1. Field of the Invention
The invention relates to communication in a orthogonal frequency division multiple access radio system.
2. Description of the Related Art
Radio systems such as a WiMAX (Worldwide Interoperability for Microwave Access) or a WLAN (Wireless Local Area Network) system may utilize an OFDMA (Orthogonal Frequency Division Multiple Access) technique where symbols to be transmitted are mapped to sub-carriers. The sub-carriers, in turn, are grouped to form sub-channels. In uplink, a time slot may have a width of three symbols and it may comprise 48 data sub-carriers, for example. A time slot also comprises pilot carriers which carry predetermined symbols for reference in the reception.
When the quality of a received signal begins to drop below an acceptable level although a user terminal is transmitting at its maximum transmission power and using the most robust encoding and modulation, failures start to appear in the reception of data packets. To be able to continue the communication, a process of re-transmission of improperly received data packets is started. After one or more re-transmissions data packets may be received without errors.
However, the re-transmission process consumes a lot of resources in the radio system. That also leads to lowered data rates and to a deteriorated service.